In the field of microelectronics there is a push to integrate photonics and microelectronics in order to improve the performance of current electronic systems. The possibility of permitting optical communication between integrated chips via an optical bus would have a major impact on the performance of electronic systems. Research in this field is very active, however proposed solutions until now have been unsuitable and are difficult to realize in practice.
Efficient coupling of highly dissimilar refractive index waveguides has always been a problem in many applications.
For example, optical fiber waveguides have been employed to convey optical signals. Optical fibers include an optical fiber core within a cladding. Optical fiber waveguide to optical fiber waveguide butt-coupling requires high precision alignment. Semiconductor waveguides typically have smaller geometries and higher refractive indices compared to optical fiber geometries and refractive indices. The geometry mismatch stems from mode confinement requirements, the higher refractive index of semiconductor waveguides demands smaller geometries. Butt-coupling between optical fibers and semiconductor waveguides require extremely high precision alignment and suffers from high coupling losses due to refractive index mismatches.
In the field of integrated opto-electronic devices, light (optical signal) coupling between “on chip” waveguides and external waveguides is conventionally done using butt-coupling between an external fiber core and the core of an on-chip waveguide or its optical interconnect device. On-chip opto-electronic devices include vertical cavity lasers, horizontal cavity lasers, photodiodes and phototransistors. On-chip optical devices include splitters and couplers.
What prevents major progress in the field is an intrinsic property of silicon, the principal semiconductor utilized in microelectronics, which has a high index of refraction (about 3.5) compared to that of optical fibers (about 1.47). The index of refraction of germanium, another important semiconductor utilized in microelectronics is about 4. This prevents efficient injection of an optical signal from one material to the other in two different ways: First, the large index of refraction difference between them causes the creation of reflections at the injection interface. Second, the required geometries of an optical fiber and of a waveguide of a high index of refraction for single mode optical signal propagation are radically different. Both of these factors reduce the ability to couple (transfer) optical signals with acceptable losses.
A similar problem exists in the field of biosensors between different materials than those employed in microelectronics however with comparable differences in refractive indices. While employing such hybrid integration technologies, an external optical signal may not be injected into a chip. However, the coupling experiences the same difficulty in injecting the signal into a layer of a high index of refraction.
On-chip waveguides typically comprise waveguides made of silicon or germanium having indices of refraction of about 3 to 4, which are much higher than the index of refraction for an optical fiber core (dielectric waveguide). For single mode fibers, core-to-core butt-coupling requires high precision alignment and superior mechanical stability, both of which add significant cost to providing such optical connections. Not only must alignment be provided, but also a good match in the waveguide properties is required to ensure that the desired modes are coupled between waveguides. In many applications, optical coupling is such a challenge that designs resort to opto-electronic conversion of signals to use electrical coupling between on-chip and off-chip optical signals.
In coupling optical signals to and from on-chip waveguides or opto-electronic devices, conventional techniques involve mounting optical fibers with precision to rest horizontally on the integrated circuit with a prism reflector being used to redirect the light vertically into a waveguide or optical device on the integrated circuit. Conventional techniques also include precision mounting of the fiber vertically on the integrated circuit. These techniques are costly to implement due to difficulties in reproducing and maintaining alignment precision.
Recently evanescent field waveguide coupling has been proposed for optical signal coupling between an optical fiber waveguide and a semiconductor waveguide.
A theoretical treatise was provided by Borges, B.-H. V. and Herczfeld, P. R., entitled “Coupling from a Single Mode Fiber to a III-V Thin-Film Waveguide via Monolithic Integration of a Polymeric Optical Waveguide”, Journal of the Franklin Institute, vol. 335B, no 1, p. 89-96, 1998. Borges describes the results of theoretical modeling of evanescent field waveguide coupling between a polymer waveguide and a sheet waveguide of unlimited extent as well results of modeling evanescent field waveguide coupling between a polymer waveguide and a sheet waveguide of unlimited extent having a step discontinuity. While the mathematical modeling seems to suggest promising coupling efficiencies, the presented results cannot be employed in practice as usable waveguides have limited widths and therefore the presented results cannot be reproduced in practical implementations. Experimental trials attempting to duplicate Borges account only for a small fraction of the promised results. An effective refractive index variation is described by Borges, at the bottom of page 90 thereof, to come from exploiting “linear and quadratic electro-optic effects, as well as plasma, band filling, and band shrinkage to yield a high figure of merit for index modulation”. Such coupling makes use of an important perturbation of the optical field at the entrance of the chip: The semiconductor upper cladding creates the perturbation and the optical field first partially couples into the upper cladding to finally transfer to the waveguide layer. That approach also creates a significant amount of radiative (lossy) mode coupling and higher order mode coupling which have not been addressed by Borges. Also, a number of assumptions have been demonstrated to be wrong in respect of structures having high refractive index differences. Furthermore, the lack of symmetry in the Borges approach prevents reverse optical signal (light) propagation from inside of the chip to the outside.
Further attempts at implementing evanescent field waveguide coupling describe tapered structures that slowly adapt the optical field of a first waveguide to match the optical field of a second waveguide. Such structures are difficult to fabricate due to a requirement for three-dimensional (3D) shaping during manufacture.
For example, one attempt at addressing the coupling problem provided tapered waveguides having large geometries in high refractive index materials. Numerous proponents have simulated such devices, and the simulations seem to suggest high optical signal injection efficiencies. Such attempts include: Dai, D., He, S. and Tsang, H.-K. “Bilevel Mode Converter Between a Silicon Nanowire Waveguide and a Larger Waveguide”, Journal of Lightwave Technology, vol. 24, no 6, p. 2428-33, June 2006; and Doylend, J. K. and Knights, A. P. “Design and Simulation of an Integrated Fiber-to-Chip Coupler for Silicon-on-Insulator Waveguides”, IEEE Journal of Selected Topics in Quantum Electronics, vol. 12, no 6, p. 1363-70, November 2006. While these simulations might suggest polarization independent solutions, only a limited number of such devices have been manufactured given the 3D nature of the devices.
Another attempt included employing a large size waveguide coupled with a reverse tapered waveguide of high refractive index described by Galan, J., Sanchis, P., Sanchez, G. and Marti, J., “Polarization Insensitive Low-Loss Coupling Technique between SOI Waveguides and High Mode Field Diameter Single-Mode Fibers”, Optics Express, vol. 15, no 11, p. 7058-65, 2007. According to this attempt, such geometry permits obtaining good injection efficiency and has little polarization sensitivity. However, problems arise from the requirement for manufacturing a suspended structure which is very fragile. Furthermore, the tapered waveguide has to have extremely small dimensions. Such implementations demand use of high resolution lithography at increased costs.
A further attempt consists using prisms integrated within chips as described by Ghiron, M., Gothoskar, P. Montgomery, R. K., Patel, V., Pathak, S., Shastri, K. and Yanushefski, K. A. in U.S. Pat. No. 7,058,261 B2, entitled “Interfacing Multiple Wavelength Sources to Thin Optical Waveguides utilizing Evanescent Coupling” and published Jun. 6, 2006 and in U.S. Pat. No. 7,020,364 B2 “Permanent Light Coupling Arrangement and Method for use with Thin Silicon Optical Waveguides” published Mar. 28, 2006. Once again, the 3D character and the numerous fabrication steps required, relegate such solutions to laboratory settings rendering them unviable for practical applications.
Yet another attempt includes employing a diffractive grating engraved directly into the high refractive index material as described by: Roelkens, G., Van Campenhout, J., Brouckaert, J., Van Thourhout, D., Baets, R., Romeo, P. R., Regreny, P., Kazmierczak, A., Seassal, C., Letartre, X., Hollinger, G., Fedeli, J. M., Di Cioccio, L. and Lagahe-Blanchard, C., in “III-V/Si Photonics by Die-to-Wafer Bonding”, Materials Today, vol. 10, no 7-8, p. 36-43, July-August 2007; Taillaert, D., Van Laere, F., Ayre, M., Bogaerts, W., Van Thourhout, D., Bienstman, P. and Baets, R., in “Grating Couplers for Coupling Between Optical Fibers and Nanophotonic Waveguides”, Japanese Journal of Applied Physics, vol. 45, no 8A, p. 6071-6077, 2006; and Taillaert, D. and Baets, R., in U.S. Pat. No. 7,065,272 B2 entitled “Fiber-to-waveguide coupler” published Apr. 26, 2005. Initial experimental results appear to show that the signal injection efficiency is relatively good and polarization independent. Also, the injection is totally independent of the “state” of the facets. Despite these advantages, the required fabrication complexity is high, because such implementations demand high resolution lithography and high precision engraving techniques. Furthermore, alignment has to be controlled very well in order to avoid optical signal losses.
Other attempts propose coupling schemes requiring nanofabrication, which can also be difficult to incorporate into CMOS chips.
There is a need in the field for increasing optical coupling efficiencies between low refractive index (dielectric) waveguides and high refractive index semiconductor waveguides.